1. Field of the Invention
The present invention relates to a voltage clamp circuit which controls an input voltage and clamps an output voltage and an integrated circuit incorporating this voltage clamp circuit.
2. Description of the Prior Art
In prior art there is a known clamp circuit comprising protective diodes and MOS transistors to control an input voltage and be prevented from erroneously operating when a large input voltage over a certain defined magnitude is applied.
For example, Japanese Patent Application Publication No. 2001-86641 discloses such a clamp circuit which comprises an input terminal to which a voltage is input, a first protective diode reversely connected between the input terminal and a first power supply terminal to which a first power supply voltage Vdd is input, a second protective diode reversely connected between the input terminal and a second power supply terminal to which a ground potential GND is input, and an MOS transistor connected between the first and second power supply terminals.
Even applied with a voltage higher than the first power supply voltage Vdd to the input terminal, this clamp circuit can be prevented from erroneously operating until the voltage exceeds a value to turn on (break down) the first protective diode, by turning on the MOS transistor to connect the input terminal and the second power supply terminal and to clamp the input voltage to about a value of the first power supply voltage Vdd.
Aiming to prevent erroneous operation when an excessive input voltage is applied, such a clamp circuit controls an output voltage in accordance with an input voltage and outputs only a voltage of a preset value from an output terminal in accordance with an applied voltage of a preset value.
However, a problem arises with such a clamp circuit when connected with devices operating by an output voltage of the circuit as a power supply since levels of voltage such devices require to operate differ. There may be a case where devices cannot normally operate by the preset output voltage of the clamp circuit.
In particular, when connected with a supply voltage supervisor IC which operates at a very low voltage, the clamp circuit is required to output a very low voltage other than the preset voltage.
Meanwhile, the input voltage also functions as a power supply to various devices but it is not always adapted to a device in connection. With an input voltage being a preset value or more, the clamp circuit may output a voltage of a value exceeding a tolerance of the device, causing the device to fail to operate.